SAN ANTONIO — Senator Clinton turned her attention yesterday to the mechanics of delivering voters to the polls in a round of primaries Tuesday that could hold the key to the future of her presidential ambitions.

Mrs. Clinton also sharpened her criticism of Senator Obama, hoping to give her backers a jolt of energy.

“His entire campaign is based on a speech he gave at an anti-war rally in 2002,” Mrs. Clinton told reporters aboard her campaign plane as she flew between events in San Antonio and Fort Worth, Texas. “The speech was not followed up by action, which is part of a pattern that we have seen repeatedly.”

It was the second day she has made national security the focus of her closing argument to voters, seeking to portray Mr. Obama as inexperienced and untested.

Mr. Obama fired back at rally in Providence, R.I., telling supporters: “Real change isn’t voting for George Bush’s war in Iraq and then telling the American people it was actually a vote for more diplomacy when you start running for president.”

Mrs. Clinton was racing between key primary states, opening in Texas and moving to Ohio, both seen as competitive contests with Mr. Obama.

“This is one of the most momentous decisions any Texan could make,” Mrs. Clinton told key activists as she launched a last-weekend blitz. “You are, in effect, hiring the next president of the United States.”

She opened her weekend by rallying backers at a training session aimed at preparing activists for the incredibly complex primary system, widely known as the “Texas two-step.” Two-thirds of the delegates at stake in Texas will be picked during a traditional primary. That’s followed Tuesday night by a round of caucuses in the state’s 8,000 precincts where the other third will be allocated.

The Byzantine process calls for a focus on grass-roots organization, and Mrs. Clinton’s campaign — boasting 40,000 volunteers in the state — was spending the weekend preparing backers for that task.

Activists from 20 counties gathered for a training session on how to take charge of those caucuses and sway voters who may be undecided. Mrs. Clinton dropped by to raise their enthusiasm, telling them they could prove the crucial difference in a breathtakingly close primary contest.

Polls have shown Texas to be a virtual dead heat and the outcome could hinge on which campaign does a better job of getting backers to the polls. Mrs. Clinton has a lead, albeit a shrinking one, in Ohio and the same dynamic is in place.

After losing 11 straight contests, many strategists argue that Mrs. Clinton must win the big tests on Tuesday to continue. Her strategists have begun to dispute that thinking and carve out a scenario for continuing if results in Texas and Ohio are disappointing.

Rhode Island and Vermont also holding contests Tuesday, but with far fewer delegates at stake.

“We’ve got to do (is) the Texas two-step,” Mrs. Clinton told cheering backers. “We’re going to take our country back.”

Mrs. Clinton planned to kick off an “88 counties in 88 hours” bus marathon through Ohio beginning tonight and stretching into Monday morning.

Speaking with reporters, Mrs. Clinton said she’ll continue to hammer on the distinctions she’s drawn with Mr. Obama on national security.

“It’s a defining issue and it’s one that the voters of Texas and America deserve to think about,” Mrs. Clinton said.

Mr. Obama has accused her of trying to scare voters. Her campaign began airing a commercial a TV ad Friday that asks voters who they want to answer the phone in the middle of the night at the White House when there’s a national security emergency somewhere in the world.